With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the process of achieving the multilayer interconnect structure with finer interconnects, film coverage of step geometry (or step coverage) is lowered through thin film formation as the number of interconnect levels increases, because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect structure, it is necessary to improve the step coverage and planarize the surface in an appropriate process. Further, since finer optical lithography entails shallower depth of focus, it is necessary to planarize surfaces of semiconductor device so that irregularity steps formed thereon fall within a depth of focus in optical lithography.
Accordingly, in a manufacturing process of the semiconductor devices, a planarization technique of a surface of the semiconductor device is becoming more important. The most important technique in this planarization technique is chemical mechanical polishing. This chemical mechanical polishing (which will be hereinafter called CMP) is a process of polishing a substrate, such as a wafer, by placing the substrate in sliding contact with a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing pad.
A polishing apparatus for performing CMP includes a polishing table that supports a polishing pad having a polishing surface, and a substrate holder, which is referred to as a polishing head or a top ring, for holding a wafer. When the wafer is polished with such a polishing apparatus, the polishing table and the polishing head are moved relative to each other while supplying the polishing liquid (slurry) onto the polishing pad disposed on the polishing table, and the wafer is pressed against the polishing surface of the polishing pad at a predetermined pressure by the polishing head. The wafer is brought into sliding contact with the polishing surface in the presence of the polishing liquid, so that the surface of the wafer is polished to a flat and mirror finish.
In such polishing apparatus, if a relative pressing force applied between the wafer and the polishing surface of the polishing pad during polishing is not uniform over the entirety of the surface of the wafer, insufficient polishing or excessive polishing would occur depending on the pressing forces applied to respective portions of the wafer. Thus, in order to even the pressing force applied to the wafer, the polishing head has a pressure chamber formed by an elastic membrane (or a membrane) at a lower part thereof. This pressure chamber is supplied with a fluid, such as air, to press the wafer against the polishing surface of the polishing pad through the membrane under a fluid pressure, and to polish the wafer.
Since the polishing pad has elasticity, the pressing force, applied to a peripheral edge of the wafer during polishing of the wafer, becomes non-uniform, and hence only the peripheral edge of the wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, a retainer ring for holding the peripheral edge of the wafer is provided so as to press the polishing surface of the polishing pad located at the outer circumferential edge side of the wafer.
A substrate transfer device, which is called a pusher, is disposed near the polishing table. This pusher has a function to elevate the wafer, which has been transported by a transporter, such as a transfer robot, and transfer the wafer to the polishing head that has been moved to a position above the pusher. The pusher further has a function to transfer the wafer, which has been received from the polishing head, to the transporter, such as a transfer robot.
In the polishing apparatus having the above-described structure, the wafer, which has been polished on the polishing surface of the polishing pad, is held on the polishing head via vacuum suction. Further, after the polishing head is elevated together with the wafer, the polishing head is moved to a position above the pusher, and the wafer is then released from the polishing head onto the pusher. Releasing of the wafer is performed by supplying a fluid into the pressure chamber to deform a wafer holding surface of the membrane.
However, if a change in the shape of the membrane is small, the wafer may not be released from the membrane. Thus, in order to ensure releasing of the wafer from the polishing head, the pusher is provided with a release nozzle, as disclosed in Japanese laid-open patent publication No. 2005-123485, Japanese laid-open patent publication No. 2010-46756, and Japanese laid-open patent publication No. 2011-258639. This release nozzle is a mechanism which ejects a jet of fluid (or releasing shower) into a gap between the wafer and the membrane to thereby assist the wafer release.
FIG. 10 is a schematic view showing a wafer releasing operation for releasing a wafer from a membrane. As shown in FIG. 10, a lower surface of a polishing head 100 is constituted by a membrane 104. When a wafer W is transported, the wafer W is held via vacuum suction on a wafer holding surface 104a which is constituted by the membrane 104. In FIG. 10, the membrane 104 is inflated so as to release the wafer W therefrom.
A pusher 150 is disposed near the polishing head 100, and the pusher 150 is provided with release nozzles 153 each for ejecting releasing shower. Specifically, the release nozzles 153 are located so as to eject the releasing shower into a gap between the wafer W and the membrane 104. A fluid mixture of pure water and N2 (nitrogen), for example, is used as the releasing shower. The jet of the releasing shower is delivered into the gap between the wafer W and the membrane 104 to thereby release the wafer W from the polishing head 100.
In order to inflate the membrane 104 so as to deform the wafer holding surface 104a, a fluid (e.g., nitrogen) having a constant pressure is supplied into the pressure chamber of the membrane 104 for a predetermined time. At this time, if the fluid is excessively supplied into the pressure chamber of the membrane 104, the membrane is largely inflated until the wafer W is brought in contact with the pusher 150, and as a result, the wafer W would be broken. Therefore, the pressure of the fluid, which is supplied into the pressure chamber of the membrane 104, is set to a relatively low pressure (e.g., about 100 hPa) so that an excess amount of the fluid is not supplied into the pressure chamber.
In contrast, if an amount of fluid supplied into the pressure chamber of the membrane 104 is insufficient, the membrane 104 cannot be properly inflated. When the membrane 104 is not properly inflated, the releasing shower does not enter the gap between the wafer W and the membrane 104, but most of the releasing shower impinges on a surface (a surface to be polished) of the wafer W. As a result, the releasing shower presses the wafer W against the membrane 104, thus inhibiting the release of the wafer W. Therefore, in order to perform the inflation of the membrane 104 with a good reproducibility, there is a demand to supply the fluid having a stable pressure into the pressure chamber of the membrane 104.
The fluid, supplied into the pressure chamber of the membrane 104 when releasing the wafer, is introduced into the polishing apparatus through a fluid main pipe 154 extending from a fluid supplying source (e.g., a fluid supply line installed in a factory) 130, as shown in FIG. 10. When the wafer is to be released, the pressure of the fluid, supplied into the pressure chamber of the membrane 104, is regulated by a pressure regulator 156 attached to a fluid supply passage 155 which branches off from the fluid main pipe 154. In the fluid supply passage 155, a valve 138 is located at a secondary side of the pressure regulator 156. When the valve 138 is opened, the fluid having a regulated pressure is supplied into the pressure chamber of the membrane 104.
The pressure of the fluid supplied from the fluid supplying source 130, is typically set to about 0.4 MPa to 0.6 MPa. In contrast, a pressure of the fluid, which is required for inflating the membrane 104, is approximately 100 hPa. Therefore, it is necessary for the pressure regulator 156 to regulate the pressure of the fluid down to about 1/40 to 1/60. However, in the pressure regulator 156 having such a wide regulation range, in many cases, a secondary-side pressure (or a downstream-side pressure) of the pressure regulator 156 would be largely affected by a change in a primary-side pressure (or an upstream-side pressure). More specifically, it is difficult for the pressure regulator 156 to supply the fluid having a stable secondary-pressure under an environment in which the primary-side pressure of the pressure regulator 156 changes.
The releasing shower is ejected from the release nozzles 153 after the pressure chamber of the membrane 104 is inflated. Since the fluid, which serves as the releasing shower, is supplied to the release nozzles 153 through the passage 158 which branches off from the fluid main pipe 154, the primary-side pressure of the pressure regulator 156 changes (i.e., decreases). Further, in order to push out water that has been collected in a gas-water separation tank disposed in a passage for attracting the wafer W, the fluid flowing in a passage 122, which branches off from the fluid main pipe 154, is used. Thus, the primary-side pressure of the pressure regulator 156 changes (i.e., decreases). The secondary-side pressure of the pressure regulator 156 also changes (i.e., decreases) in accordance with the change in the primary-side pressure. As a result, the membrane 104 cannot be properly inflated.